There presently exist electronic devices such as vidicons which use an electron beam to detect the presence of optical or electronic signal patterns on a recording surface. Depending upon the particular application, these devices use different means for focusing and deflecting the electron beam, as well as for collecting the signal currents representative of the electronic pattern recorded on the recording surface All of these devices have three things in common, namely (1) a cathode which emits the electron beam, (2) a recording surface which is able to sense an optically induced electronic resistance pattern, and (3) collector means for collecting a signal current resulting from the scanning of the optically induced pattern by the beam.
In a typical vidicon, the recording surface is a transparent metallic plate on the face of the vidicon tube, one side of which is coated with a thin layer of photoconductor material. The optical image is focused onto the photoconductor material layer and is scanned with an electron beam originating at a cathode spaced from the tube face while the optical image persists. The scanning beam deposits electrons on each scan spot so as to generate a current to ground whose magnitude corresponds to the change in resistance of the photoconductor which, in turn, is proportional to the intensity of illumination on said spot The current through a load resistor connected to the plate, which constitutes the output of the vidicon, therefore, reproduces the variations in the light intensity of the successive portions of the optical image projected onto the face of the vidicon.
Prior apparatus such as this for reading an image from an electronic recording surface without making it visible via electrotatic toner, but using a scanning electron beam instead, are disadvantaged because the positions of the electron beam source and the recording surface position are fixed. Accordingly, only one image at a time can be stored on the recording surface. In other words, in order to read an image from that surface, all previous images recorded thereon will have had to be stored elsewhere so that the recording surface can be erased. Therefore, electron devices such as vidicons have limited application as means for reading electronically stored images.
There have been some efforts in the past to develop a recording medium or phototape which can store a plurality of electronic images for later readout directly using a scanning electron beam. Such a system is disclosed in the publication Electrostatic Imaging and Recording by E. C. Hutter, et al, Journal of the S.M.P.T.E. Vol. 69, January 1960, pp. 32-35. The recording medium or phototape in that reference is also disclosed in U.S. Pat. No. 3,124,456 (Moore). The medium comprises a transparent polyester base coated on one side with a layer of photoconductive material which is, in turn, coated with a thin layer of a dieletric material. To record an image on that medium, the dielectric layer is precharged by a voltage applied across that layer and then the photoconductive layer is exposed to a light image while an electric field is applied across the dielectric layer. The charge in the dielectric layer decays towards zero with the decay being most rapid where the optical image is brightest and, therefore, the photoconducitve resistance is lowest. After a time corresponding to the greatest difference between the potentials in the light and dark areas of the medium, the electric field is turned off and the discharging process stops, thereby leaving on the dielectric layer an electrostatic charge distribution corresponding to the optical image incident on the medium. The stored image may be read from the medium by scanning the dielectric layer with a focused electron beam to produce an electrical signal corresponding to the stored image.
The Hutter et al system has several disadvantages. It can store acquired data in the recording medium for only a limited period of time, e.g. a few weeks, because of charge leakage in the dielectric layer of that medium Also, the medium has poor light sensitivity. Accordingly, the quality of the images recorded on the medium is not very high. Most importantly, readout of the image stored on the medium by the scanning electron beam is accomplished by detecting a capacitively modulated current signal from the medium involving simultaneous movement of many charge carriers in the medium. Resultantly, the resolution of the detected picture signal is much less than that of the stored electronic image which, as just stated, was fairly poor to begin with.
The Hutter et al system is disadvantaged also because in the process of reading the stored images on the recording medium using a scanning electron beam, those images are rapidly degraded by the electron beam scanning process itself because the beam electrons cause electronic conduction in the medium. In other words, when that system performs a read operation, it also tends to erase the images stored in the medium. This, of course, is completely unacceptable if that arrangement is to be considered for storage of images which may have to be retrieved several times during the storage period. Also, during the image sensing operation, the photoconductive medium proposed by Hutter is plagued by excessive dark currents which reduces the signal detection threshold sharply.
Other systems which use a scanning electron beam to read images on a movable storage medium are disclosed in U.S. Pat. Nos. 3,880,514 (Kuehnle) and 4,242,433 (Kuehnle et al). Those systems are superior to the Hutter et al recording system in that they do not require precharging of the recording medium; otherwise they are disadvantaged in the same respects. Indeed, all these prior systems are totally useless for long term storage of high resolution image patterns and for sensing signals at very low light levels.